The invention relates to fine-celled polyolefin foams, which are suitable for use in the packaging sector, for motor vehicle parts and for components of electric and electronic equipment.
The production of polyethylene foams, which are not cross linked, from high-pressure polyethylene as polyethylene extrusion foams (WO 87 04445) or polyethylene particle foams by the extrusion method (EP 0 505 850) or by the suspension impregnation method (EP 0 212 204) is known. The long-chain branchings, contained in the high pressure polyethylene, bring about a high melt strength of the polyethylene and, as a result, a melt of high stability in the foaming step.
The low dimensional stability at elevated temperature and the low stiffness are disadvantages of polyethylene foams. Compared to polyethylene foams, polypropylene foams have a higher dimensional stability at elevated temperatures and a higher stiffness. However, because of the low melt strength of the linear polypropylene, the processing of polypropylenes into polypropylene foams requires expensive technological measures or a modification of the polypropylene. It is not possible to process standard propylenes on conventional foam extruders.
For the production of extrusion foams using standard polypropylenes, carbon dioxide as a blowing agent and nucleating agents, the foamed sheet must be pulled off by special take-off rollers heated to 95.degree. C. (Dey, S., SPE-ANTEC 1996, 1955-1958).
To achieve a uniform cell structure in particle foams using the suspension impregnation method and unmodified polypropylenes, expensive technological measures are required, such as treating the reactor contents with a gas at the bottom of the reactor during the discharging process (below the stirrer or through the stirrer) (EP 0 630 935) or adding a heating fluid during the emptying of the autoclave until 3/4 of the particle foam has been discharged (EP 0 290 943). High shrinkage is a further problem of particle foams, which were prepared from unmodified polypropylenes using butane as a blowing agent. By an expensive treatment of the shrunken particles at a temperature 20.degree. below the softening temperature, under compressed air or a compressed inert gas, the shrinkage can be reversed (EP 0 687 709).
It is furthermore known that polypropylene blends and modified polypropylenes can be processed on normal, standard polyethylene foam extruders into extrusion foams and extrusion particle foams.
Known blending components for polypropylene during the manufacture of extrusion foams on normal, standard polyethylene foam extruders are polyethylene (British patent 2,099,434), EVA (British patent 2,099,434), butyl rubber (European patent 0 570 221), styrene-butadiene rubber (U.S. Pat. No. 4,467,052), as well as (meth)acrylate copolymers (U.S. Pat. No. 5,506,307). However, these blending components, which are not coupled, lead to a strong decrease in the dimensional stability at elevated temperatures, as well as in the stiffness and the compression strength of the polypropylene foams.
Known modified polypropylenes, which are suitable for the production of extrusion foams and extrusion particle foams, are polypropylene, modified by peroxides capable of polymerizing (WO 94 05 707) and polypropylenes, modified by high energy electron beam radiation (EP 0 190 889, De Nicola, A., Polymeric Materials Science and Engineering (1995), 106-108).
The very high costs of the safety measures for the biological shielding of the high-energy radiation are a disadvantage of the method of modifying polypropylene by high-energy electron radiation. These safety measures make it necessary to shield the irradiation chamber hermetically by means of thick special concrete walls or lead segment constructions, to employ an expensive measuring technique as protection against radiation and to medically monitor the operating personnel.
It is an object of the present invention to develop fine-celled polyolefin foams of high dimensional stability at elevated temperatures and stiffness with foam densities ranging from 10 to 600 kg/m.sup.3, while avoiding the disadvantages of the known foam formulations.